The present invention relates to centrifugal turbomachines, to centrifugal impellers for turbomachines and to the related production methods, particularly, but not exclusively, for oil and gas applications.
A centrifugal turbomachine is a rotary machine where mechanical energy is transferred between a working fluid and a centrifugal impeller. In oil and gas application, where the fluid is typically a gaseous fluid, centrifugal turbomachines include compressors and expanders. A compressor is a turbomachine which increases the pressure of a gaseous fluid through the use of mechanical energy. An expander is a turbomachine which uses the pressure of a working gaseous fluid to generate mechanical work on a shaft by using an impeller in which the fluid is expanded.
In uncompressible fluid, e.g., water, centrifugal turbomachines include pumps and turbines, which transfer energy between the fluid and the impeller in a way analogous to compressors and expanders, respectively.
In general, in all cases, the working fluid exchanges energy with the centrifugal machine by flowing in the centrifugal impeller along a radial outward direction, oriented from an axis of rotation of the impeller to a peripheral circumferential edge of the impeller.
In particular, the centrifugal impeller of a compressor turbomachine transfers the mechanical energy supplied by a motor that drives the turbomachine to the working gaseous fluid being compressed by accelerating the fluid in the centrifugal impeller. The kinetic energy imparted by the impeller to the working fluid is transformed into pressure energy when the outward movement of the fluid is confined by a diffuser and the machine casing.
Centrifugal turbomachines can be fitted with a single impeller, in which case they are frequently referred to as single stage turbomachines, or with a plurality of impellers in series, in which case they are frequently referred to as multistage centrifugal turbomachines.
A prior art embodiment of a multistage centrifugal compressor 100 is illustrated in FIG. 1, in an overall section view, and in FIGS. 2 and 3, in more detailed section views. Compressor 100 is included in a casing 102 within which is mounted a shaft 101 and a plurality of impellers 110. The shaft extends along an axis of rotation X of compressor 100. The shaft 101 and impellers 110 are included in a rotor assembly 103 that is supported through a couple of bearings 150 and 160, which allow the rotor assembly to rotate around the axis of rotation X. The multistage compressor 100 comprises a plurality of stages 107 (seven stages 107 in the embodiment in FIG. 1), each stage 107 including one impeller of the plurality of impellers 110 and a portion of the casing 102, where an inlet duct 170 upstream the impeller 110 and an outlet duct 180 downstream the impeller 110 are provided. The impeller 110 has a typical closed design configuration including an impeller hub 113, which closely encircles the shaft 101, and a plurality of blades 108 extending between a rear impeller disc 114 and a front shroud 119. The impeller 110 comprises an inlet low-pressure side 111 defined by an impeller eye 115 on the front shroud 109 and an outlet high-pressure side 112 defined by a peripheral circumferential edge of the impeller 110.
Through operation of the impeller 110, each stage 107 of the multistage compressor 100 operates to take an input process gas flowing along the inlet duct 170, to drive the gas from the inlet low-pressure side 111 to the outlet high-pressure side 112 of the impeller 110 and to subsequently expel the process gas through the outlet duct 180 at an output pressure which is higher than its input pressure.
The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof.
An impeller eye seal 120 is provided between the impeller eye 115 of each centrifugal impeller 110 and the casing 102, in order to prevent the fluid from leaking in the space between the casing 102 and the impeller 110, from the outlet high-pressure side 112 to the inlet low-pressure side 111. The casing 102 includes an inlet ring 104 facing the impeller eye 105 and provided with a cavity for housing the impeller eye seal 120.
The impeller eye seal 120 is of the labyrinth type with a plurality of teeth 121a-e (five teeth 121a-e in the embodiment in FIGS. 1-3). Each tooth 121a-e extends radially towards the axis of rotation X and circumferentially around the axis of rotation X. The envelope profile of the teeth 121a-e is conical in shape with a mean diameter 122. The eye seal 120 is mounted on a housing in the casing 102 and placed in such a way that a first tooth 121a toward the inlet low-pressure side 111 is smaller in diameter than a last (fifth) tooth 121 e toward the outlet high-pressure side 111. To match the shape of impeller eye seal 120, the impeller eye 115 is provided with a stepped region 116 comprising a plurality of steps 117a-e (five steps 117a-e in the embodiment in FIGS. 1-3), each facing a respective tooth of the plurality of teeth 121a-e. The plurality of steps 117a-e includes a first step 117a toward the inlet low-pressure side 111 having a diameter 123a which is smaller than the diameter 123e of a last (fifth) step 117e toward the outlet high-pressure side 112 of the impeller 110.
Fluid leakages through the eye seal 120 must be reduced as much as possible for the reason that the portion of fluid leaking from the outlet to the inlet side has to be compressed again through the impeller, thus reducing the efficiency of the turbomachine.
An impeller having the same design of impeller 110 can be used also in an expander, the main difference being the fact that the gaseous fluid expands in the impeller, i.e., the inlet side, corresponding to the impeller eye, is the high-pressure side while the outlet side, corresponding to the peripheral circumferential edge is the low-pressure side. In an expander the impeller eye seal prevents the fluid from leaking in the space between the casing and the impeller, from the inlet high-pressure side to the inlet low-pressure side. Fluid leakages through the eye seal must be reduced as much as possible also in an expander, for the reason that the portion of fluid leaking from the inlet to the outlet side does not flow through the impeller and therefore does not contribute to generate mechanical work on the shaft, thus reducing the efficiency of the turbomachine.
It would be desirable to design and provide an improved sealing system for reducing the leakage flow through the impeller eye of a centrifugal impeller.